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Abstract

The variation in the depth and width of the median valley along the Mid-Atlantic
Ridge (MAR) suggests that the formation of ocean crust at slow spreading centers is not a
simple two-dimensional process in which crustal accretion occurs uniformly both along the
ridge axis and with time. Rather, it has been proposed that the ridge axis can be divided
into a number of distinct segments or spreading cells. This thesis investigates the
segmentation model by studying the variability in the structure and tectonics within
spreading cells at 23°N and 26°N along the MAR. The results support the segmentation
model in which accretion varies along the ridge, evolving as independent spreading cells or
segments, with different portions of the ridge system being in different stages of volcanic
and tectonic evolution.
Chapter 2 presents an overview of morphologic and tectonic variations along a 100-
km-length of the MAR south of the Kane Fracture Zone (MARK area). Sea MARC.I side
scan sonar data and multi-beam Sea Beam bathymetry are used to document the distribution
of crustal magmatism and extensional tectonism near 23°N. The data indicate a complex
median valley composed by two distinct en echelon spreading cells which overlap in a
discordant zone that lacks a well-developed rift valley or neovolcanic zone. The northern
cell, immediately south of the fracture zone, is dominated by a large constructional volcanic
ridge and is associated with active high-temperature hydrothermal activity. In contrast, the
southern cell is characterized by a NNE-trending band of small fissured and faulted
volcanos that are built upon relatively old, fissured and sediment-covered lavas; this cell is
inferred to be in a predominantly extensional phase with only small, isolated volcanic
eruptions. Despite the complexity of the MARK area, volcanic and tectonic activity appears
to be confined to the 10-17 km wide inner rift valley. Small-offset normal faulting along
near-vertical planes begins within a few kilometers of the ridge axis and appears to be
largely completed by the time the crust moves out of the median valley. Mass-wasting and
gullying of scarp faces, and sedimentation which buries low-relief seafloor features, are the
major geological processes occurring outside the rift valley. In Chapters 3 and 4, the microearthquake characteristics and P wave velocity
structure beneath the median valley of the Mid-Atlantic Ridge near 26°N are studied; this
ridge segment is characterized by a large high-temperature hydrothermal field situated
within the inner floor at the along-axis high. Chapter 3 explores the tectonic variations
within the crust as evidenced from the distribution and source mechanisms of
microearthquakes observed by a network of seven ocean bottom hydrophones and two
ocean bottom seismometers over a three week period in 1985. Hypocenters were
determined for 189 earthquakes, with good resolution of focal depth obtained for 105
events. Almost all events occurred at depths between 3 and 7 km beneath the seafloor,
with earthquakes occurring at shallower depths beneath the along-axis high (<4 km). The
distribution of hypocenters and the diversity of faulting associated with earthquakes
beneath the inner floor and walls suggests a spatially variable tectonic state for the ridge
segment at 26°N. These variations are presumably a signature of lateral heterogeneity in the
depth region over which brittle failure occurs, and are a consequence of along-axis changes
in the thermal structure and state of stress.
We suggest that at present the hydrothermal activity and deposition of massive
sulfides is being sustained by heat generated by a recent magmatic intrusion. A
consequence of this scenario is that thermal stresses play a dominant role in controlling the
distribution of earthquakes and nature of faulting. Such a hypothesis is consistent with an
apparent lack of seismicity beneath the hydrothermal field, the location of hypocenters
around the low velocity zone (Chapter 4), attenuation of P wave energy to instruments atop
the high (Chapter 4), the higher b-values associated with the along-axis high region, and
the occurrence of high-angle (or very low angle) normal faulting and reverse faulting, as
well as the variability in nodal plane orientations, associated with inner floor events beneath
the along-axis high and the volcano. In Chapter 4, we report results from the explosive refraction line and from the
tomographic inversion of P wave travel time residuals for seismic velocity structure in the
vicinity of the hydrothermal field. The twcrdimensional along-axis P wave structure
beneath the inner floor indicates that young oceanic crust cannot be adequately characterized
by a simple, laterally homogeneous velocity structure, but that one-dimensional StruGtures
are at least locally valid (at 5-10 km length scales). The shallowmost crust (upper 1-2 km)
beneath an axial volcano and the along-axis high is characterized by significantly higher
velocities (by more than 1 km/s) than are associated with the upper crust in the deepest
portions of the median valley. The variation is inferred to be a consequence of more recent
magmatic and volcanic activity in the along-axis high region, as compared with the alongaxis
deep where tectonic fissuring has created a highly porous crust characterized by lower
seafloor velocities. The crust beneath the along-axis deep appears to be typical of normal
young oceanic crust, with a mantle velocity of 8.25 krn/s observed at 5 k:m depth.
A low velocity zone centered beneath the along-axis high and extending under an
axial volcano is imaged from 3 to 5 km depth (7.2 km/s to 6.0 km/s); the velocity decrease
is required to satisfy the travel time residual data and to explain the severe attenuation in
compressional wave energy to instruments atop the along-axis high. The presence of an
active high-temperature hydrothermal field atop the along-axis high, together with the
observations of lower P wave velocities, the absence of microearthquake activity greater
than 4 km in depth, and the propagation of S waves through the crust beneath the volcano
and along-axis high (Chapter 3), suggest that the volume corresponds to a region of hot rock with no seismically-resolvable pockets of partial melt. The shallow velocity gradients
describing the low velocity volume(<0.6 s-l) appear to be a corrunon characteristic of
inferred zones of magmatic intrusion on the MAR. Comparison of the depth to the velocity
inversion with the depths determined in other seismic studies at locally high regions along
the MAR, the Juan de Fuca Ridge, and the East Pacific Rise reveals a correlation between
lid thickness and spreading rate, suggesting that the amount of magma available at each
location is spatially variable, or that the differences in lid thickness are describing the
temporal evolution of magmatic intrusions beneath mid-ocean ridges.
In Chapter 5, the first direct measurement of upper mantle P- and S-wave delay
times beneath an oceanic spreading center is presented. Two independent estimates of the
epicenters and origin times are made for each of two earthquakes in a 1985 earthquake
swarm near 25°50'N on the Mid-Atlantic Ridge using local and teleseismic arrival time
data. Comparison indicates a 14-20 km northward bias in the epicenters teleseismically
located using a Herrin [1968] Earth model. The bias is due to departures of the actual
velocity structure from that implicit in the travel time tables used for the locations,
combined with unbalanced station distribution. The comparison of origin times for the
best-located event, after correction for the epicentral bias and for an oceanic crustal
thickness, shows there to be only slightly lower velocities than a Herrin [1968] upper
mantle; the P-wave delay is +0.3 ± 0.9 s (+0.2 ± 0.9 sand -2.4 ± 0.9 s relative to the
isotropic Preliminary Earth Reference Model (PREM) and the Jeffreys-Bullen [1940] (JB)
travel time tables, respectively). The lack of a resolvable P-wave delay suggests that the
Herrin [1968] model is a good approximation to the average upper mantle velocity beneath
this segment of the MAR.
Measurement of the S-wave delay for the same MAR swarm event shows there to
be a positive delay (+3.1 ± 2.0 s), or larger travel times and slower velocities compared to
the JB S-wave tables (+ 3.9 ± 2.0 s relative to the isotropic PREM S-wave model). In
contrast to the larger P-wave delays found in other MAR studies, the lack of a significant
seismic anomaly near 26°N indicates that sizeable regions of low velocity material do not
presently exist in the upper few hundred kilometers of mantle beneath this section of the
ridge. This evidence argues for substantial along-axis variations in the active upwel~ng of
mantle material along the slowly-spreading Mid-Atlantic Ridge. In order to explain the
observation of a smaller than expected P wave delay in a region where the S delay suggests
significant temperature anomalies (low velocities), we propose a model for mantle
upwelling in which the decrease in travel time is due to an anisotropic P wave structure
(fast direction vertical); the anisotropy results from the reorientation of olivine crystals
parallel to the ascending flow and balances the travel time delay due to a region of low
velocities.

Description

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1990

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